In fields of medical diagnosis, industrial inspection, security and the like, inspection using a radiation inspection apparatus such as a tomograph (X-ray CT apparatus), is performed. The X-ray CT apparatus usually contains an X-ray tube (X-ray source) for irradiating fan beam X-rays, a fan-shaped X-ray beam, an X-ray detector opposed to the X-ray tube and having many X-ray detection elements, and an image reconstruction apparatus for reconstructing an image based on data send from the X-ray detector. An object is placed between the X-ray tube and the X-ray detector, and a cross-sectional plane is imaged by irradiation with fan beam X-rays.
The X-ray CT apparatus repeatedly performs an operation of irradiating fan beam X-rays and collecting X-ray absorption data, with changing an irradiation angle to the cross-sectional plane in turn, for example, by 1 degree at a time. Then, the X-ray CT apparatus analyzes the obtained data, calculates an X-ray absorptance of the object on the cross-sectional plane, and constructs an image of the cross-sectional plane according to this absorptance by a computer.
A solid scintillator for radiating visible light and the like by stimulation of X-rays is used as the X-ray detector of the X-ray CT apparatus. The solid scintillator means a scintillator containing ceramic or a single crystal, among scintillators.
When the solid scintillator is used as the X-ray detector of the X-ray CT apparatus, it is preferable that the detection elements are miniaturized, and it is easy to increase the number of channels, and therefore, higher resolution is possible.
Conventionally, as a solid scintillator used for a radiation detector such as an X-ray detector, for example, single crystals, such as cadmium tungstate (CdWO4), sodium iodide (NaI), and cesium iodide (CsI), barium fluoride chloride:europium (BaFCl:Eu), lanthanum oxybromide:terbium (LaOBr:Tb), cesium iodide:thallium (CsI:Tl), calcium tungstate (CaWO4), cadmium tungstate (CdWO4), gadolinium oxysulfide:praseodymium (Gd2O2S:Pr) disclosed in Japanese Patent Laid-Open No. 58-204088 (Patent Document 1), and the like have been known.
Among these solid scintillators, rare earth oxysulfide ceramics such as Gd2O2S:Pr, have a large X-ray absorption coefficient to allow miniaturization of the solid scintillator, and provide a short afterglow time of light emission and therefore have high time resolution. Therefore, rare earth oxysulfide ceramics is preferable for a scintillator for X-ray detection and a rare earth oxysulfide ceramics scintillator is widely put to practical use.
However, in recent years, a scintillator that has short afterglow, can perform high speed scanning, and has high light output has been desired so as to reduce an X-ray exposure amount of a patient.
Conventionally, as solid scintillators having short afterglow, garnet structure oxides using Ce3+ of rare earth as a light-emitting ion have been known.
For example, Japanese Patent Laid-Open No. 2005-126718 (Patent Document 2) proposes garnet structure oxides such as (Tb1-y, Cey)a(Al, Ga, In)zO12 and (Lu1-y, Cey)a(Al, Ga, In)zO12, and International Publication No. WO 99/33934 (Patent Document 3) proposes a garnet structure oxide such as (Gd1-x, Cex)3Al5-yGayO12.
As a method for manufacturing these solid scintillators, a method for making a sintered body by a hot pressing (uniaxial pressing) method, an HIP method (hot isostatic pressing method), or a vacuum sintering method is used.